Tool for material-removing machining

ABSTRACT

A cutting insert is formed with a cutting edge, a cutting corner at an end of the cutting edge, a cutting surface, and a row of raised scales extending along the cutting edge. Each scale is formed with an arcuate apex line transverse to the cutting edge and forming at the cutting edge relative to a perpendicular to the cutting edge an angle between 55° and -5°, a steep flank to one side of the apex line, and a shallow flank to the other side of the apex line. The apex line is concave toward the other side.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of PCT applicationPCT/DE95/00515 filed 12 Apr., 1995 with a claim to the priorities ofGerman applications P 44 15 491.7 and P 44 31 796.4 respectively filed 2May 1994 and 7 Sep., 1994.

FIELD OF THE INVENTION

The invention relates to a tool for material-removing machining ofworkpieces, in particular a cutting insert, with a plurality of recessedand/or raised chip-shaping elements which extend along a cutting edge ina row and which are elongated with respective longitudinal axes, thecutting surfaces having at least near the cutting edge a scale-likemicrostructure and the individual scales being arranged immediatelyadjacent one another. The invention also relates to a boring toolequipped with the described cutting inserts.

BACKGROUND OF THE INVENTION

Such a tool is for example described in German patent document4,118,068. There the adjacent chip-shaping elements are arranged atvarious angles from a maximum of 45° at the cutting-corner regions to 0°relative to perpendiculars to the cutting edge, various traveldirections of the chip during the cutting operation being taken intoaccount. Above all only one of the effective chip-shaping elements canbe optimally oriented relative to the chip travel direction. The otherchip-shaping elements not properly positioned relative to thechip-travel direction create material deposits and substantial frictionto a degree dependent on the variation of the orientation of thechip-forming element from the actual chip-travel direction. This is alsotrue for a tool according to German patent document 4,118,070 whosechip-shaping elements are arranged parallel to one another along thecutting edge.

German patent document 3,105,377 describes a cutting insert whosecutting surface has a plurality of spaced, adjacent, and upstandingprojections or recesses, the spaces between the projections or in thecutting surface holding at least one row of generally semicircular orkidney-shaped recesses extending parallel to the cutting edge and onlyslightly spaced from each other. The longitudinal axes of the recessesof the row are parallel to the respective cutting edge. A chip movinghere thus undergoes extreme frictional resistance which also leads todepositing of workpiece material and material adhesions and/or highcutting temperatures. The same is for true tongue-like projectionsextending from the cutting-surface plateau toward the cutting edgeaccording to U.S. Pat. No. 5,265,985.

OBJECT OF THE INVENTION

In workpieces for material-removing machining, in particular forturning, milling, or boring, which are used with little or no coolantand by means of which soft or highly alloyed or hardened or laminatedworkpieces are machined, it is an object to reduce as much as possiblethe local friction with different chip cross sections, that is withdifferent chip-travel directions and/or curvatures in order to reduce oreliminate detrimental workpiece adhesions or bonding to the cuttingsurface.

SUMMARY OF THE INVENTION

This object is solved by a tool having at least near the cutting edge ascale-like microstructure and the individual scales are arrangedimmediately adjacent one another and each have parallel to the cuttingedge a section which has a steep flank and a shallow flank, and the apexlines of the scales extend at different angles and/or curvatures. Theflat(ter) flank is on the side of the apex line toward the center ofcurvature. In this manner with any cutting width of the chip passingover the contact zone the angle of the apex lines of the scales isgreater than or equal to the respective span-travel angle and/or thecurvature of the apex lines is equal to or greater than the respectivelateral chip curvature. The flank angles reduces the chip contactsubstantially on the flat flank near the apex line, so that if necessarythe chip can slide unhindered over the apex lines. In particular withindexable cutting plates for turning and for corner milling the apexlines of the scales are curved and arranged at different angles. Incutting inserts that are used for boring, the curvature of the apexlines is more important than the orientation of the scales at variousangles. The apex lines of the scales form relative to the cutting edgewith a perpendicular to the cutting edge on the cutting surface an anglebetween 55° and 5°.

Thus preferably the apex lines for turning and milling cutting insertsare directed away from the cutting corner or for boring cutting insertsthe curvatures are directed at the drill axis so that they follow thepath of actual chip-travel direction. Preferably the curvatures from thecutting corner to the cutting-edge center or to the next cutting corneror from the effective cutting corner to the adjacent cutting-edge cornerare so shaped that with turning or milling tools they have increasingradii and by boring tools (due to the differently oriented chip traveldirection) decreasing radii from cutting corner to cutting corner. Thecurvature of the apex line of an individual scale is preferably constantover its length, lying according to a particular embodiment of theinvention between 0.1 mm and 12 mm near a cutting corner to 2 mm to 60mm increasing at the cutting-edge center, with increasing spacing fromthe effective cutting-edge corner. This is true for a substantial partof the actual scales.

According to the cutting operation the maximum height of the scalesmeasured as a spacing of the apex line from the foot point of a flanklies between 0.01 mm and 0.25 mm, preferably between 0.025 mm and 0.1mm.

To better guide the chip the height of the scales increases withincreasing distance form the cutting edge.

The apex lines of the scales or their tangents extend at a cutting angleof -10° to +30° and/or form with respective perpendiculars to thecutting edge from the cutting corner to the cutting-edge center adecreasing angle of 55° to -5°.

According to a further embodiment of the invention the length of thescales increases from the cutting corner to the cutting-edge center orto the next cutting corner along a cutting edge in regions or steadily,lying preferably between 0.15 mm and 8 mm. The flanks of the scales arein section straight, rounded, or concave, preferably with radii ofcurvature >0.1 mm for steep flanks and >1.5 mm for flat flanks. Theflanks of the scales or their tangents extend relative to the cuttingedge on the side turned away from the center of curvature of the apexlines under a steeper angle of 45°±30° and on the other side under aflatter angle <15°. The transitions between adjacent flanks of thescales at the apex lines is sharp or rounded.

In particular with polygonal cutting inserts with multiple cuttingcorners, in particular with left- and right-cutting indexable cuttinginserts according to the invention in the region of the cutting cornersthere is a scale extending in the direction of an angle bisector andhaving an apex with in cross section perpendicular to the angle bisectorat least the same height as the apex lines of adjacent scales and whoseflank angle (in contrast to the adjacent scales) is the same, the apexangle, that is the angle formed by the flanks, being 130° to 175°,preferably 150° to 170°. The apex angle of the scale decreases withincreasing spacing from the cutting edge either in parts steadily orsuddenly.

The scales can extend right to the cutting edge or run out at thecutting edge, the scale height dropping to 0 mm. Alternatively it isalso possible between the cutting edge and the scales for there to be aland or a chip-shaping inlet, if necessary the scales forming part ofthe chip-forming inlet or merging therewith. The scales can be arrangedat a land.

Preferably tools for turning are formed with a positive or neutral freeangle, with a one- or two-sided construction, straight or curved edgesor as a (rectangular) milling plate.

The number of scales lies between 3 and 30, preferably 10 through 25,with the scales at least in the region of the effective cutting edge,that is in the chip-engaged region.

The above-described invention principles can also be realized withcutting inserts having a raised central surface, a so-called cuttingsurface plateau. This cutting-surface plateau, that projects past thecutting edge plane or through a chip-shaping plane formed by the apexlines, has preferably in the region closer to the cutting edge roof-likescales which are formed in accordance with the above-described scales,preferably differing therefrom by shorter length.

As a result of the above-discussed construction a milling tool isproduced that with any cutting width in the chip contact zone the angleof the apex lines of the scales is greater than or equal to the actualchip-travel angle and/or the curvatures of the apex lines is greaterthan or equal to the actual lateral chip curvature. In connection withthe flank inclinations the chip contact is largely reduced woo the flatflank near the apex, if necessary the chip can slide unhindered over theapex lines. In particular when corner milling the apex lines of thescales are curved and at different angles.

In order to optimize the known milling tool with respect to thecutting-corner construction of the planing cutter, that is in order toreduce as much as possible the local friction with different chipsections so that deleterious material adhesions and depositions arereduced or prevented on the cutting surface, near a cutting corner ornear a planing edge there is at least one scale with the same curvaturedirection as the scales on the main cutting edge and having a curvaturedirected away from the cutting corner. As a result of this formation ofthe planing cutter sections near the cutting corner the chip shaping isalso improved in this region. This means with multiple-edged workpiecesthat with right-cutting milling plates the above-described scaleformation is provided at one cutting corner, preferably in the region ofthe actively cutting edge of the cutting surface. In the back regions ofthe same cutting edge which is used as a planing cutter after turning ofthe indexable cutting plate in the tool holder there is at least onescale which relative to the scales at the farthest cutting corner has adifferent shape. This scale has the same curvature direction as thescales as those of the adjacent cutting corner of the main blade whosecurvatures are directed away from the cutting corner.

Preferably the radius of curvature of the scale(s) on the planing edgeis the same as or bigger than the radii of curvature of the scales whichare near the cutting edge of the main cutting edge.

The angle formed between the end of the scale and a perpendicular fromthe main cutting edge toward the planing edge is equal to or bigger thanthe corresponding angle of the scales which are near the cutting edge ofthe main cutting edge. The scale closest to the cutting corner orindividual scales on the planing edge end at a spacing or terminate nearthe cutting edge at a spacing of two verticals which are determined onthe one hand by the known scale end near the plane-cutting edgeperpendicular to the plane cutting edge and on the other hand by thecontact point of the planing edge and the cutting edge perpendicular tothe plane cutting edge. The spacing of these verticals is in the rangeof the minimal tooth advance which is determined by the advance inmillimeters per revolution divided by the number of cutting edges andthe product of the maximum number of teeth and the maximum toothadvance. This spacing lies preferably between 0.03 mm and 4 mm. Thescale or scales on the planing edge end at a spacing from the planingedge which is equal to one to four times the planing error, preferablyin the range 0.01 mm and 0.5 mm.

In particular the above-described cutting inserts are used in boringtools of the type described in claim 26. Such boring tools are describedin principle in German patent document 4,018,043. Within the scope ofthe present invention these boring tools have cutting inserts of thetype described in claims 1 through 20 with the feature that the radii ofcurvature of the apex lines of these cutting inserts are directed at thedrilling axis, preferably from the cutting corner radially to theadjacent cutting corner and getting smaller toward the drilling axis.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the tool according to the invention are described belowwith reference to the drawings. Therein:

FIG. 1 is the chip movement with the cutting insert according to theinvention;

FIG. 2 is the chip movement with a cutting insert according to the priorart during a cutting operation similar to that of the tool of FIG. 1;

FIG. 3 is a top view of a tool according to the invention;

FIGS. 3a, 3b, 3c, and 3d are sections taken along respective linesIIIa--IIIa, IIIb--IIIb, IIIc--IIIc, and IIId--IIId of FIG. 3;

FIGS. 3e and 3f are large-scale sectional views of the details indicatedat respective circles IIIe and IIIf of FIGS. 3a and 3b, respectively;

FIGS. 3g and 3h are large-scale views of the details of the cuttingedges as indicated at respective circles IIIg and IIIh of FIG. 3;

FIGS. 4a through 4f are partial sections through the cutting edges oftools according to the invention;

FIGS. 5a and 5b are side and top views of an indexable milling cuttingplate according to the invention;

FIGS. 6a and 6b, 7a and 7b, and 8a and 8b are side and top views offurther indexable milling cutting plates according to the invention;

FIGS. 9a, 9b, and 9c are front side, end, and back side views of aboring tool fitted with two cutting inserts;

FIGS. 10, 11, and 12 are top views of cutting inserts intended forboring;

FIG. 13 is a top view of a milling tool according to the invention; and

FIG. 14 is a section taken along line XIV--XIV of FIG. 13.

SPECIFIC DESCRIPTION

As seen in particular in FIGS. 3a-3h as well as in FIGS. 5b, 6b, 7b, and8b the cutting plate according to the invention has on its cuttingsurface 10 along the cutting edge 11 individual adjacent scales 12 whichare elongated with longitudinal axes that extend at various anglesinward from the effective cutting corner 13 to the cutting-edge center14 relative to the cutting edge. Relative to a respective perpendicularto the cutting edge the angles nearer the cutting corner are larger andbecome smaller toward the cutting-edge center, becoming if desired 0 ornegative. In addition to the various angular positions the scales 12 arealso formed so that their flanks are curved arcuately seen from thecutting corner along their longitudinal direction. Each scale 12 canhave different curvatures along the longitudinal direction at differentlocations or can be of constant curvature over the entire length. In thecase of FIGS. 3a-3h the curvature increases toward the regions remotefrom the cutting corner. The cutting insert can have as shown in FIG. 3a central plateau 15 or a central recess. The cutting insert can be madewith or without a mounting hole 16.

FIGS. 1, 3a, 3e, and 3f show scales that have an apex point 121 forminga border between a steep flank 122 and a shallow flank 123, the steeperflank 122 lying on the side turned away from the center of curvature ofthe apex line.

The flanks can be formed either planar or slightly convex or concave(thus part spherical). Similarly the apex line 121 can be sharp-edged orrounded; the same is true for the transitions 124 from the flatterflanks 123 to the steeper flanks 122. Finally the two enlarged sectionalviews of FIGS. 3e and 3f show that the flanks can have in the regionsclose to the cutting edge different angles from in the regions remotefrom the cutting edge. In this case the angle α of the flatter flank 123increases away from the cutting edge while the angle β of the steeperflank 122 remains generally the same. In each case the angle β is largeralong the entire length of the respective scale.

In the embodiment of FIGS. 3a-3h the individual scales 12 have a greaterheight away from the cutting edge, that is a larger spacing of the apexline 121 from the foot point 124 (see heights H_(A) and H_(B) in FIGS.3e and 3f.

In addition the cutting insert has in the cutting corner 13 a raisedchipping element formed mirror symmetrical relative to a respectiveangle bisector, so that the respective scale 17 widens conically towardthe center of the cutting surface. The scale 17 which is shown alongsection lines IIIc--IIIc and IIId--IIId of FIG. 3 in FIGS. 3c and 3d hastwo symmetrical flanks 171 which form an angle between 150° and 170°.This angle is smaller close to the cutting edge than remote therefrom.The apex line 172 in every case is above the apex lines 121 of adjacentscales 12. The cutting insert according to FIG. 3 has along each cuttingedge eleven scales 12 which extend from the cutting corner 13 along thecutting edge 14.

The action of the cutting insert according to the invention is clearlyshown in FIG. 1. The passing chip 18 is gently guided over the flatlyextending flank 123 so that it does not contact the sharp-edge steepflank. In contrast thereto in the cutting insert 20 of the prior art,which has longitudinal ribs extending at various angles to the cuttingedge, the passing chip 19 must overcome considerable friction which canlead to workpiece adhesions or deposits 21 (FIG. 2). This is true since,even in an embodiment where the longitudinal ribs have differentdirections relative to the cutting edge, only one longitudinal ribextends in the chip-travel direction while all the remaining ribs 22impart movement components to the passing chip 21 which are transverseto the longitudinal direction of the respective ribs.

As in particular shown in FIGS. 3g and 3h, the curvatures of the apexlines 121 point in the same direction while they decrease from thecutting corners to the cutting-edge center. With a presumedpart-circular formation the radii of curvature go from 0.1 to 12 mm atthe cutting corner 12 to 2 through 60 mm in the cutter center. This istrue for turning and milling tools. For the cutting insert according toFIG. 3 the corresponding radius change shown in FIG. 3g, which shows thedetail E, is clearly shown in comparison to FIG. 3h, which shows thedetail F. The changing radius of curvature decreases with the angle nuefrom the region close to the cutting corner to the region remote fromthe cutting corner.

FIG. 4a shows a section taken vertically to the cutting edge where theflank 122, the apex line 121, and the transition line from the flank 122to the adjacent flank 123, the line 124, is visible. With the embodimentaccording to FIG. 4a the scales 12 reach right to the cutting edge andform the chip-guiding inlet. With the embodiments according to FIGS. 4band 4d the scales 23 and 24 extend to the cutting edge with the scales23 being formed arcuate perpendicular to the cutting edge and formingindividual steps or inflection points. The cutting surface plane isrecessed with increasing spacing form the cutting edge. The travel ofthe scales 24 according to FIG. 4d is linear and descending, that is ata constant cutting angle relative to the free surface.

With the embodiment according to FIG. 4c the scales 25 are constructedsuch that they end at the cutting edge 26, that is their height isreduced to 0. The cutting surface at first rises from the cutting edge26 but then drops linearly in regions remote from the cutting edge. Thecutting insert according to FIG. 4e has a land 27 adjacent which thescale formation 28 is formed. The scale apex line 281 lies slightlybelow the plane of the land 27. FIG. 4f shows a cutting insert where thescales 12 lie in the region of a land 31. The cutting insert shown inFIG. 4f is made of a very hard material and is intended for machiningextremely hard workpieces.

FIG. 5 shows an indexable milling cutting plate and FIGS. 6 through 8shows indexable cutting plates for turning.

The indexable cutting plate according to FIG. 5 has a generally rhombicshape seen in top view with scales extending along the cutting edgesextending from the sharp corners and having in the cutting corner alength L₂ which increases to a greater length L₁ and remains constantwhen spaced about a third of the way along the cutting edge from theeffective cutting corner. Otherwise the scales according to FIG. 5 areformed like those of FIGS. 2 and 3.

The indexable cutting plate according to FIG. 6 is also rhombic and hasfour cutting edges of the same length. To each side of a cutting cornerwhich forms an acute angle there is a row of scales which extends pastthe respective cutting-edge center as also described with reference tothe embodiment of FIG. 3. In addition the cutting insert has furtherscales 29 on the central cutting-surface plateau 30. These scales 29have however a substantially shorter length than the scales 12 at thecutting edge or a land there. As shown in FIG. 6a the free angle of thecutting plate according to the invention can be positive or 0°corresponding to FIG. 7a.

The indexable cutting plate according to FIG. 8 is square and has a freeangle of 0°.

FIG. 9a shows a right-turning boring tool in a top view of the outercutter, FIG. 9b shows this boring tool in an end top view, and FIG. 9cshows a right-turning boring tool in top view on the inner cutter.Boring tools of this type are used for boring in solid metal and have ashaft 32 and several, here two, similar cutting inserts 33 mounted onthe end of the shaft at various radial spacings and 180° offset fromeach other, with their working regions overlapping and with the radiallyinner indexable cutting plate with its engaged cutting edge extendingsomewhat past the bit axis 34. Such a bit is described for example inGerman patent document 4,018,043 to which specific reference is made.Instead of the cutting inserts described there with two adjacent cuttingedges inclined at an obtuse angle to each other it is possible as shownin FIGS. 9a and 9c to use rectangular or rhombic cutting inserts.

Such cutting inserts are shown for example in FIGS. 10 and 11. FIG. 10shows a square cutting plate in a double-edged configuration, that isopposite sides have a scale-like microstructure. FIG. 11 shows acorresponding cutting plate in a four-edged embodiment wherein thescales extend in this embodiment to the cutting corners 35.

FIG. 12 shows a trigonal indexable cutting plate where during boring twocutting sections 36 and 37 of a cutting edge between adjacent acutecutting corners 38 are used. The resultant chip passes over theintervening obtuse cutting corner 39. With this embodiment the radii ofcurvature of the apex lines of the acute cutting corners 38 decrease tothe adjacent acute cutting corner.

As can be seen in FIG. 13 the milling tool according to the invention,here shown as a square cutting plate, has on its cutting surface 10along the (main) cutting edge 11 respective adjacent scales 12 which areelongated and which have longitudinal axes that are arranged at variousangles from the effective cutting corner 13 to the cutting-edge centerand beyond relative to the cutting edge. Relative to respectiveperpendiculars to the cutting edge the angles are greater close to thecutting edge corners and become smaller toward the cutting-edge center.The scales 12 are curved on their respective flanks seen from thecutting edge along their longitudinal direction with the curvatureincreasing away from the cutting edge or constant over the entirelength. The cutting insert can be formed with or without a mounting hole16.

With the thus formed right-cutting milling plate the curvatures aredirected away from the cutting corner 13 along the main cutting edge 11.

In addition this milling plate has in the region of the planing edge 11aat least one further scale 12a with the same radius of curvature as thescales 12b on the main cutting edge. The curvature with a radius R_(PS)of this scale 12a on the planing edge 11a is at least as large as theradius of curvature of the scales 12b on the cutting edge near thecutting corner. The angle nue_(PS) on the scales 12a formed by a tangentto the scale direction relative to the cutting edges 11a is at least aslarge as the corresponding angle of the scales 12b below which these runrelative to perpendiculars 11 to the main cutting edge. The planing edge11a starts at point E_(PS) where the cutting corner 13 which is definedby a corresponding corner radius ends and merges into the straightcutting edge 11a which extends away from the cutting corner at thecutting edge 11. The spacing of the point E_(PS) from the point at whichthe scale 12a ends on the planing cutting edge 11a is shown at c. Thisspacing is between 0.03 mm and 4 mm. The scale 12a extends relative tothe cutting edge 11a at a spacing d which lies in the range of 0.01 mmand 0.5 mm. In the present embodiment only one scale 12a is provided butthere could also be two or more adjacent such scales of the samecurvature.

FIG. 14 is a section along line A--A of FIG. 13 which shows that theindividual scales 12 (as well if necessary as the scales 12a) lieimmediately adjacent one another.

We claim:
 1. A cutting insert formed with:a cutting edge; structure forming a cutting corner at an end of the cutting edge; a cutting surface; and a row of raised scales extending along the cutting edge and each in turn formed withan arcuate apex line transverse to the cutting edge and forming at the cutting edge relative to a perpendicular to the cutting edge an angle between 55° and -5°, a steep flank to one side of the apex line, and a shallow flank to the other side of the apex line, the apex line being concave toward the other side.
 2. The cutting insert defined in claim 1 wherein the other side is directed away from the cutting corner.
 3. The cutting insert defined in claim 1 wherein the other side is directed toward a drilling center axis.
 4. The cutting insert defined in claim 1 wherein the apex lines are of increasing radii of curvature away from the cutting corner.
 5. The cutting insert defined in claim 1 wherein when used for drilling the apex lines are of decreasing radii of curvature from cutting corner to cutting corner.
 6. The cutting insert defined in claim 1 wherein radii of curvature of the scales vary from between 0.1 mm and 12 mm at the cutting corner to between 2 mm and 60 mm at a center of the cutting edge.
 7. The cutting insert defined in claim 1 wherein the scales have a height measured from the respective apex lines to bases of the respective flanks equal to between 0.01 mm and 0.25 mm.
 8. The cutting insert defined in claim 1 wherein the scales have heights measured from the respective apex lines to bases of the respective flanks that increase away from the cutting corner.
 9. The cutting insert defined in claim 1 wherein the apex lines extend at a cutting angle of -10° to +30°.
 10. The cutting insert defined in claim 1 wherein the scales have measured perpendicular to the cutting edge predetermined lengths which increase away from the cutting corner.
 11. The cutting insert defined in claim 10 wherein the lengths lie between 0.15 mm and 8 mm.
 12. The cutting insert defined in claim 1 wherein the steep flanks are curved with a radius of curvature >0.1 mm and the shallow flanks are curved with a radius of curvature >1.5 mm.
 13. The cutting insert defined in claim 1 wherein the steep flanks form with the cutting surface an angle of between 15° and 75° and the shallow flanks form a flatter angle smaller than 15°.
 14. The cutting insert defined in claim 1 wherein the scales are rounded at the respective apex lines.
 15. The cutting insert defined in claim 1, further comprisinga chip-shaping scale at the corner generally symmetrical to a bisector of the corner and forming an angle open away from the corner of between 130° and 175°.
 16. The cutting insert defined in claim 15 wherein the angle decreases away from the cutting corner.
 17. The cutting insert defined in claim 1 wherein the scales extend to and end at the cutting edge.
 18. The cutting insert defined in claim 1 wherein the insert is further formed witha substantially planar land between the scales and the cutting edge.
 19. The cutting insert defined in claim 1 wherein there are between 3 and 30 scales along the cutting edge.
 20. The cutting insert defined in claim 1 wherein the cutting insert is further formed witha second row of chip-shaping scales extending along the cutting edge, the first-mentioned row of scales being between the second row and the cutting edge.
 21. The cutting insert defined in claim 1 wherein the flanks are substantially planar.
 22. The cutting insert defined in claim 1 wherein the cutting insert is further formed witha side planing edge extending transversely to the cutting edge from the corner and constituting the structure forming the corner, and a further scale at the corner concave in the same direction as the scales along the cutting edge.
 23. The cutting insert defined in claim 22 wherein the further scale has a larger radius of curvature than the scales along the cutting edge.
 24. The cutting insert defined in claim 1 wherein the insert is polygonal.
 25. The cutting insert defined in claim 1 wherein the cutting edge is straight. 